The presently disclosed subject matter relates to a biological signal measuring system, and more particularly to a system for identifying the concentration of a substance in blood (hereinafter, such a substance is referred to as “in-blood substance”) based on a biological signal acquired from a subject that is an example of the living body. The presently disclosed subject matter relates also to a biological signal measuring apparatus which is to be used in the system.
Measurement of the degree of oxygenation of blood is important for determining whether blood is sufficiently supplied to the living tissue or not. The degree of oxygenation of arterial blood, i.e., the arterial oxygen saturation can be easily measured by using a pulse oximeter. There is also a method of measuring the blood concentration of carbon monoxide hemoglobin COHb or methemoglobin MetHb, which is abnormal hemoglobin, by using the principle of the pulse oximeter (for example, see JP-A-2002-228579).
When the principle of the pulse oximeter is used, non-invasive measurement can be easily performed, but the pulse wave amplitude depends on the measurement accuracy. In the case where a disorder occurs in the circulation and the pulse wave extremely weakens, for example, the measurement is disabled. In the case of carbon monoxide poisoning, when the COHb concentration exceeds 50%, the cardiac function is lowered and comatose is caused. Also in such a case, measurement based on the principle of the pulse oximeter is sometimes impossible.